Well, the Macs usually launch on Tuesdays, so that leaves 4 possible dates:
11/6
11/13
11/20
11/27
Thanksgiving falls on the 22nd this year and I would imagine Apple wants to get these out before Black Friday, aka the start of Christmas shopping time. Based on my theory, I would guess either the 13th or 20th. Again, this is just an estimated guess.